Is it legally, ethically, morally wrong for one person to take over a character and storyline created by someone else and take it ahead?

In this particular case, I think it is. In this particular case, I wish this book had never been written.

Why? Let’s list out the reasons:

The murder of the character of Lisbeth Salander: Not literally, though that would’ve been better. The Lisbeth we were introduced to and grew to love was a mystery. A simple person with a very clear black & white view of the world, she had few requirements and despite being a little socially awkward, was someone you would be okay hanging out with you in the same room.
The Lisbeth we meet in this book is portrayed a genius certified, a hacker of unmatchable merit in the entire fricking world. A hacker who belongs to a bunch of hackers (loosely indicating Anonymous). A hacker who takes on the NSA for reasons that are mentioned in a long-winded manner.
The Lisbeth in this book is unravelled and laid out in one short book for the reader’s ‘enjoyment’. And by that one simple act, the author destroys a beloved character forever.

The weakening of Blomkvist: You know this man as a good reporter in a world where there are so few. And in this book, he is a completely weak, confused man who doesn’t have a say in anything. But he is so intelligent, so keen about Salander that he heads out on a wild goose chase or close enough to search for her.
*Spoiler Alert*

Bad writing: The first 3 books kept you up through the night, wondering what would happen next. It was fast-paced, intriguing and a beautiful web. This book is a bad attempt at ‘plugging the gaps’ or so it seems. The author is hell bent on explanations, because the earlier pages require those explanations, so it gets quite long winded. And then confusing. And just tedious.

Crappy Plot: Sorry, but there are no other words to say it. The author has taken every little bit of news about hacking that he came across in the past few years – Edward Snowden, Anonymous and their exploits, NSA’s all-seeing eye, some corporate espionage – and tried to put it all into the book. So you have the NSA, you have a hacker’s group (to which Salander belongs), you have a game developer and some corporate espionage. And then the attempt to tie it all together.
Then, the author remembers that Lisbeth never does anything without an actual reason that matters to her. Or so he thinks. So instead of developing a good villain, he fishes in the old pool of characters, and gets Salander’s SISTER as the villain. That naturally means more back story and more tedious text.
Sure, you can say that roots of all of these were their in the earlier books but sometimes, when you end a series, it is for good reason.

This entire book is written simply to make money. And it comes at a high price – the death of a beloved character in recent times.